I haven't heard a lot about Marek being an exceptional duelist - just that he's good with the Force and needs the Force to take people in duels.
There is only one book with him from his own perspective, of couse you wouldn't hear.
Marek trained on droids like Maul as well as sparred with Vader from childhood. He drilled saber skill as much as Force and book shows that well. In all fights he demonstrated flawless saber skill trying to outwit his opponets by changing styles as well as recognizing their styles and tactics. There is no implication whatsoever that he lacks saber skill or speed in any way.
Originally posted by Rookwood
Apparently Vader's speed is diminished, thanks to the iron-lung he walks around in, but Lumiya doesn't have the same situation as he does, so her speed and mobility doesn't suffer.
Vader uses Force to overcome his limitations. Yoda has far more limitations, yet, it didn't prevent him from going toe to toe with most powerful Sith in sabers.
Apart from overwhelming amount of speed feats there is even easier comparison. Anakin and Kenobi matched in speed. Logically Vader would be slower than Kenobi. Yet, they confront again and Vader chooses to fight with lightsaber to demonstrate how he improved since their last encounter.
"Last time I was nothing but the learner and now I am the Master".
You can make countless logical arguments, they will mean nothing without proofs. Indeed his suit limits him from certain things. He lacks mobility, yet, his original style doesn't rely on it. He can't dodge as good as before but with Makashi which allows swinging saber around more freely and immense strength that comes with suit he can afford to block all the attacks. He overcame his limitations and became as deadly as he ever was.